Light-sensitive silver halide photographic materials are comprised of hydrophilic colloid component layers containing various photographic additives. These photographic additives are generally incorporated in the hydrophilic colloid compositions for forming component layers by dissolving them in water or in watermiscible organic solvents and adding the resulting solution to the colloid compositions.
Generally, however, many photographic additives are difficult to dissolve in water and, even when soluble in water-miscible organic solvents, they can be incompatible with hydrophilic colloid compositions when incorporated therein with said organic solutions. This is the case of many photographic additives which are rendered non diffusible within the photographic layers by including a long hydrophobic carbon atom chain (a so called hydrophobic (ballasted) photographic additives are, for example, dye-forming couplers, DIR compounds, UV absorbers, antioxidants, image stabilizers, etc.
Typically, the process of incorporating such hydrophobic photographic additives into hydrophobic colloid components layers of photographic materials, such as silver halide emulsion layers, protective layers, intermediate layers and the like, consists in incorporating into hydrophilic colloid coating compositions of said layers the photographic additives in the form of a dispersion of fine droplets consisting of a water-immiscible high boiling organic solvent in which hydrophobic additives have been dissolved.
According to the dispersion technique, the hydrophobic photographic additives generally are dissolved in water-immiscible high boiling organic solvents (also called in the art permanent solvents, crystalloidal solvents, oil-type solvents, oil-formers and the like) and the resulting organic solution is added to an aqueous composition containing a hydrophilic colloid (gelatin) and a dispersing agent (surfactant). The mixture is then passed through a homogeneizing apparatus (colloidal mill) to form a dispersion of fine droplets of said organic solvent comprising the hydrophobic photographic additives. In some cases it may be advantageous to facilitate the dissolution of the additives by use of an auxiliary water-immiscible low boiling organic solvent, which is removed afterwards by evaporation, as described e.g. in U.S. Pat. Nos. 2,801,170; 2,801,171; 2,949,360 and 2,835,579. The obtained dispersion is then mixed with the hydrophilic colloid composition (gelatin silver halide emulsion or other gelatin-containing composition) which is used to form (by coating) the photographic layer.
Processes and organic solvents for dispersing photographic additives are well known in the art, as disclosed for example in U.S. Pat. Nos. 2,322,027, 2,801,171, 2,949,360, 3,554,755, 3,748,141, 3,779,765, 4,353,979, 4,430,421 and 4,430,422.
Organic solvents for dispersing hydrophobic photographic additives are required to meet several needs. They have to possess an excellent dissolving power towards said additives, are not to cause crystallization of additives, must keep the fine droplets stably dispersed, have a refractive index which is as close as possible to that of the hydrophilic colloid werein they are dispersed, and not deteriorate the physical properties of the layers wherein they are incorporated. Moreover, said organic solvents must not negatively affect the photographic properties of the photographic materials wherein they are used to disperse photographic additives. For example, they must not give rise to fogging of the light-sensitive silver halide emulsions, not negatively affect the stability during storage of dye-forming couplers (dispersed with said organic solvents) and of the dyes formed from said couplers during processing (stability to heat, humidity and light).
Accordingly, there is a continous need for providing improved water-immiscible high boiling organic solvents for use in photography to disperse hydrophobic additives and for providing improved techniques for incorporating said additives into photographic layers.